


A tale of three brothers

by Arcy



Category: Undertale (Video Game)
Genre: Bad Puns, For Science!, Gen, Implied Human Experimentation, Not actual science, Prequel, Sans-centric
Language: English
Status: Completed
Published: 2016-08-15
Updated: 2016-10-09
Packaged: 2018-08-08 20:13:51
Rating: Teen And Up Audiences
Warnings: Major Character Death
Chapters: 7
Words: 19,205
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/7771600
Author URL: https://archiveofourown.org/users/Arcy/pseuds/Arcy
Summary: <blockquote class="userstuff">
              <p>Long before the events in Undertale, Sans and Papyrus had a third brother. The enigmatic W.D. Gaster, royal scientist. Some time ago Sans stop working as his lab assistant. What will it take for the brothers to once again join forces? And is that even a good idea?</p><p>A story of people doing the right things for all the wrong reasons and very wrong things for all the right reasons.<br/>This can't possibly end well.</p>
            </blockquote>





	1. Gaster

**Author's Note:**

> Here we go! My first ever fanfic. I am made of excitement right now.
> 
> So for clarity's sake: This fic takes place before the events in Undertale. I know there's a ton of stories out there that have Gaster as Sans and Papyrus' father/creator, but I wanted to try a different route. In fact, as soon as I first heard about Gaster, this was the story that popped up in my mind.
> 
> Hopefully you'll enjoy reading this as much as I enjoy writing it.

Sans and Papyrus didn’t have too many next-door neighbours. The reason for this was abundantly clear this morning, as a voice rang out, startling a couple of birds that had made the unfortunate decision to spend the night on the roof.

“SANS! SANS! WAKE UP SANS! TODAY’S THE DAY!”

Sans came out of his room, rubbing his eyes. “I’m up, Paps, I’m up. Whoa, what time is it? Are you sure it’s legal to be up this early?”

There wasn’t much of a day-night cycle in the underground. Yet there were stars, of a sort. Crystals, moss and certain creatures that glowed more brightly depending on the time of day. The soft blue glow that currently lit up the house reminded Sans of all the times he’d spent working late on some theory or calculation. He'd hoped that after he quit working at the lab he'd never have to see this particular time of night again.

“THERE’S NO TIME TO WASTE! HE’S COMING TO VISIT AND LOOK AT THE STATE OF THE HOUSE!” Papyrus narrowed his eyes. “OR DID YOU FORGET?”

“I uh…” Sans had in fact forgotten. He blinked as Papyrus leveled an accusing finger at him.

“I KNOW THAT LOOK! YOU DID FORGET! WHAT AM I GOING TO DO WITH YOU?”

“Dunno bro. You could always…” Sans got a mischievous glimmer in his eyes. “Put me under a rest?”

Papyrus groaned, as Sans knew he would. “UGH, THIS IS NO TIME FOR PUNS! HELP ME CLEAN UP THE HOUSE. I DON’T WANT OUR BROTHER TO SEE IT IN THIS STATE.”  
He proceeded to grab a broom and started furiously, yet efficiently, sweeping dust out of the house.

Sans watched him for a while, before going into the kitchen and grabbing a bucket and mop. He even cleaned a few of the whoopie pillows he left scattered around the house as a matter of principle.  
Truth be told, he couldn’t care less about the state the house was in when Gaster arrived. Why should he care what their older brother thought of them? But Papyrus cared. And that was enough reason for Sans to care a little too. Even if only grudgingly.

***

The arrival of the royal scientist, fabled creator of the Core that powered all of the underground, caused quite a ruckus in Snowdin town. People flocked to their windows and watched in awe as the tall, gangly skeleton strode up to the house of the skeleton brothers and knocked. He was dressed in a long, dark overcoat that left a trail in the snow behind him.

Papyrus opened the door as soon as Gaster knocked, almost as if he’d been waiting on the other side; which in fact he had. He then swooped his brother up in a tight hug, lifting the frail scientist a few centimeters off the ground. Gaster looked profoundly uncomfortable, but Papyrus didn’t seem to notice.

“IT’S GOOD TO SEE YOU BROTHER! IT’S BEEN SO LONG! NO, EVEN LONGER THAN THAT.”

“Uh… Good day. Papyrus.” Gaster said in his soft, distorted voice, as he brushed the snow off his coat. I see you’re in good spirits. as usual.” An unpractised, but warm smile crept up on his face as he looked at his brother with genuine fondness. The smile quickly faded, however. “And where is. Sans?”

At the mention of his name, Sans came down the stairs, his usual grin plastered on his face. It didn’t reach his eyes.

“Heya doc, long time no see.”

“Sans.” Gaster gave a curt nod. He took the small house in at a glance. “Quite a change. from your house in New Home. How do you enjoy. living in the country?” His tone made it clear what he thought about their current place of residence. Papyrus’ face fell and Sans could feel anger flare up, even if it never affected his smile.

“Oh you know, it could be worse. Good food, better company. I don’t miss working for you at all.”

Gaster’s tone turned almost wistful. “Your talents are wasted here. We could use you back in the lab. Alphys is. a fine assistant. But she is not as good as you.”

“Yeah, well maybe you should’ve thought of that, before...” Sans bit back the rest of his words.

Papyrus looked from one brother to the other, feeling the temperature in the room drop a few degrees. He had to do something. And he had to do it fast. He quickly put an arm on Sans’ shoulder and another one around Gaster.

“WELL, BROTHER! NOW THAT YOU HAVE REJOINED US, THERE IS NOTHING WE THREE CAN’T DO. AND SO FOR OUR FIRST CHALLENGE, THE GREAT PAPYRUS - THAT’S ME, BY THE WAY - HAS PREPARED SOME BREAKFAST-SPAGHETTI-AND-EGGS. I CALL IT: BREGGHETTI! WE SHALL CONQUER IT AND IT WILL BE MARVELOUS.”

He could feel the tension in Sans shoulders lessen. “Alright Pap. I could use some eggcercise.” He softly chuckled to himself. Gaster’s face remained a blank. He never did get Sans’ puns. Sans had often wondered if the scientist had a sense of humor at all. So far all evidence pointed to the contrary.

Gaster obligingly followed them into the kitchen, even though there was some trepidation in his voice as he said: “It’s been a while since I sampled your cooking. Papyrus. It will be quite. the experience. I’m sure.”

***

Breakfast was indeed marvelous. Papyrus did most of the talking, something which suited the other two brothers fine. When he got a word in, Gaster shared some news about New Home and even tried some of the bregghhetti. He swallowed heavily and Sans could see his eyes watering, but he took another bite all the same. Papyrus beamed. Sans felt his anger subside a bit. This was almost like old times. Almost. Papyrus used to make Gaster food all the time back then. He wondered if the scientist missed it. Gaster did look thinner than usual. Whispier. As if he wasn’t all there.

After breakfast was done, Papyrus insisted he wash the dishes by himself so that Sans and Gaster could “catch up”. He looked so hopeful at them that Sans didn’t have it in him to argue.

And so Sans and Gaster ended up sitting on the couch in a mutually awkward silence.  

“How are you? ” Gaster signalled eventually, in the sign-language shorthand he’d always loved to use in the lab. He claimed it was more efficient. Talking was a nuisance.

Sans would have none of it today.  
“Hey, I've got an idea. How about you use your words?”

Gaster looked displeased, but to his credit made the effort to speak. “So how is. whatever it is you do here?”

Sans shrugged. “Oh, the usual. Teaching the local kids at the library, selling hotdogs, working on some stuff… Nothing that would interest you. How about you?” For a moment the light in his eyes flickered. He knew he shouldn’t, but he couldn’t keep himself from adding:  
“ _Murder anyone lately_?”  
  
He kept his voice low. Papyrus didn’t know the reason Sans had stopped working at the lab, nor was he aware of the nature of the experiments that took place there. As far as Papyrus was concerned, Gaster might’ve just been working on a new and exciting flavor of nicecream. Sans intended to keep it that way.

Gaster seemed taken aback, but not as much as Sans would have liked. “Sans. They’re not like us. They’re not monsters. They. are. humans.”

“Oh, I dunno. That human the king sent to us? The one with the ribbon? She seemed like a _child_ to me.”

“The king needs. souls.”

“Well, then let him do his own dirty work. I’m done.”

“We could use you. Need you.” A pause. “I need your input. your brilliance. In some ways. it outshines. even mine.”

Sans crossed his arms and leaned back. “Done.” he repeated with emphasis. He did feel some mild surprise. Gaster was usually big on ego and sparse on compliments. He must want Sans back badly to go through such depths of flattery.

Gaster sighed. A heavy sound that seemed to travel through his entire body. “If you change your mind. when you change your mind. you will be welcomed back. home.”

“Yeah? Don’t count on it doc.” Sans threw a fond glance at the small house and the snow that had begun to fall outside of the window.  
“This is home now.”

***

_At that precise moment, just as the first fresh snowflakes touched the roof of the skeleton brothers’ house; a door opened somewhere in the forest. It was a large door and heavy. The small figure had to push hard to get it to open. Then the crack was wide enough and the figure slipped through. Step by careful step it began to walk. Away from the ruins. Leaving very small, very human footprints that were soon wiped away by the snow._

 


	2. Human

***

  
Much to Papyrus’ disappointment and Sans’ relief, Gaster left early the next morning. But not before trying to persuade Sans to come back with him one more time. It had stopped snowing and the temperature outside was pleasantly cool.

“You and Papyrus. both. I will find you a place to stay. You will like Alphys. She is odd. in ways that I am not.”

“Didn’t we already have this discussion yesterday?” Sans looked weary. He hadn’t slept much. Gaster being in the house brought back too many memories.

“You will like our latest project. brother. If successful. we might not need any more human souls. at all.”

This did get Sans’ attention. He had to fight back the urge to ask more questions. “No.” He said, shaking his head. ”I know you. Last time you told me I would “like” your project, we ended up working on a weapon so dangerous it’s probably a good thing it never worked properly.” He raised a bony eyebrow. “You haven’t…”

Gaster raised a hand in assurance. “I have not progressed. with that particular project. You and I remain the only two. who know of..” he allowed himself a small smile “the Gaster blaster”.

“Only you would name a deadly weapon after yourself…”

“You must admit, Sans. You did like having access to that. particular. magic.”

Sans allowed his grin to widen for perhaps a fraction of an inch. He hated it when Gaster was right. “Maybe a little. It was kinda fun, while it lasted.” In the end they had only managed to summon a blaster once. It had taken days to patch up the hole in the lab. He shrugged. “Still not coming back to work for you.”

“You will change. your mind.” Gaster said with that arrogant confidence that Sans found so intolerable about him.

“Don’t wait for it.”

Gaster didn’t respond. He merely turned away to say goodbye to Papyrus who was coming out of the house to give his brother another bone-crushing hug.

“IT’S A SHAME YOU HAVE TO LEAVE SO SOON.” Papyrus said. Sans thought he looked genuinely lonely at that moment. It was with a slight pang that he realized Papyrus might actually be lonely.

While Sans was pretty much at ease anytime and anywhere, Papyrus thrived in the company of others. And he hadn’t made many friends in Snowdin. For the first time Sans wondered if he’d made the right choice in leaving New Home. He’d basically forced Papyrus to choose between his brothers. They’d never been apart before. Papyrus wouldn’t dream of letting Sans go to a new place by himself. But Snowdin was tiny. Perhaps too tiny for his awesome, big mouthed, bigger hearted brother.

“Yes, Papyrus.” Gaster said. “It is a shame. Because I did enjoy my time. here. Truly.” His mouth twisted into the semblance of a smile. “And your cooking. But work calls.” He avoided Sans’ gaze.

“Gaster?” Sans said, just as Gaster had begun to walk away.

“Yes. Sans?” 

Sans held out his hand.

“Stay safe.”

Gaster took the outstretched hand and a long whinded PWEEEEEEEEEEEEEPPPPPPP erupted from the whoopie cushion Sans had hidden beneath.

Sans doubled over, laughing, while Papyrus scowled at him and Gaster merely looked confused.

“That is never not funny.” Sans managed between bursts of mirth.

  
***  


“I’M GOING TO THE STORE SANS, DO YOU NEED ANYTHING?

“Naw, that’s ok bro.”

Now that Gaster was on his way home, Sans could safely move all the junk he had stored behind the house. No one ever looked behind the house. Not even Papyrus. It sure was a lot of work having guests over. He didn’t mind doing some work, but his brother sure drove him hard. Sometimes he wished he could just take it easy for a while.

There was an old table he’d been meaning to fix up. An assortment of spare parts for Papyrus’ action figures. And of course his pile of  whoopie cushions. Sans tossed the one he’d just used on top.

There was a CRACK behind him from a snapping branch. Sans spun around. And found himself face to face with a … human. The human startled at seeing him and fell backward, into the bushes it had come from. Startled, Sans took a few steps back. He stumbled over the cushions, landing right in the middle of the pile.  
The sound that followed was glorious. Like a mighty fog horn that had something stuck in its throat and was struggling to get it out. It went on for a long time.  
  
Two bright eyes peeked at Sans from the bushes. Sans tried to struggle upright, but only managed to squeeze the last of the air from the cushions. There was a giggle coming from between the leaves.

“Heh.” Sans couldn’t help himself. It was pretty funny. “Heh heh. Heh heh hahahahaha.” He grasped his stomach. The human was laughing too. He (Or was it a she? It was so hard to tell with humans.) had stepped out of the bushes and held his sides with both hands.

They stayed like that for a while. Skeleton and human, laughing and laughing, because every time Sans tried to stand, one of the cushions would make a sound, sending them into renewed bouts of mirth. Finally, finally, Sans managed to get to his feet.

“Wow kid, you startled me.” he said, not unfriendly. Ok, so this was a human. And humans were dangerous. But then anyone who could laugh like that couldn’t be all bad.

“I’m sorry. Just… too funny.” the boy said, still grinning. His hair was the color of straw, with a gleam of orange, sticking out from underneath what looked like a cowboy hat. He had a small bandana wrapped around his neck and a messenger bag slung over one shoulder.  
His already large eyes widened even further when he finally calmed down enough to get a better look at Sans. “What… I mean who are you?”

“The name is Sans. Pleased to meetcha.” Sans held out his hand.

The boy hesitated for a second, then slowly shook it. “Cody. He said. That’s my name.” The name sounded somewhat familiar to Sans, but he couldn’t remember where he’d heard it before. He shrugged, it was probably nothing.

There was another sound and for a moment Sans thought one of the cushions still had some air left afterall. But then Cody put a hand on his stomach and it was clear that this was a different kind of rumble.

“You hungry?” Sans asked, feeling a pang of pity. He couldn’t take his eyes off this kid. He looked so much like that other human… The one that Gaster… No, that was not a train of thought he cared to pursue. Ever. “Come on, Papyrus cooked enough food for a week. I’m sure we can find you something.”

The kid looked a bit wary, as if fearing Sans was going to lead him into a trap. Sans couldn’t blame him. It wasn’t an entirely unrealistic fear.  
  
“I don’t want to impose...”

“Nah. It’s the least I can do.” He’d get him some food, then sent him on his way. Sooner rather than later, really. It was still a human. And humans meant trouble.

  
***  


Humans sure ate a lot. Cody wolfed down anything Sans put in front of him. It was a good thing Papyrus had gone off to get more groceries, They’d need more food.

  
As if thinking of his brother had summoned him, both Cody and Sans looked up as they heard footsteps in the snow outside. And Papyrus was not alone. Two voices could be heard, talking animatedly. Someone was keeping pace with Papyrus’ conversation. Sans would’ve spent more time marveling about this, if he hadn’t had a more pressing concern.

“Quick kid, hide!” He said. He looked around for a conveniently shaped lamp, chair, anything. But nothing seemed to match Cody’s silhouette.

Cody had needed no prodding however. Before Sans could say more, he jumped over the couch and crouched down behind it. Kid was nimble, he had to give him that.

“Sure, yeah, that’ll work.” he mumbled.

Not a second too soon as Papyrus flung the door open with even more gusto than usual.

“SANS!” he announced happily.

“Huh, yeah, bro?” Sans said, looking up from the newspaper he’d been pretending to read. Fortunately Papyrus was too preoccupied to notice Sans was holding it upside down.

“I AM PLEASED TO ANNOUNCE THAT I MET SOMEONE NEW DURING MY EXPEDITION TO THE STORE. SANS, MEET UNDYNE. UNDYNE, SANS.”

“You made a friend? That’s great!.” Sans was genuinely happy, yet his smile strained when a huge, blue fish-woman came inside, wearing the uniform of a royal guard.

Her grin was huge and teehty. “Your brother has told me much about you on our way down from the store!” She shook his hand with such force he could feel his bones clatter.

“SHE’S GOING TO TEACH ME SOME ADVANCE SPAGHETTI MAKING SKILLS!” Papyrus beamed. He added in what he probably thought was a conspiratorial whisper, but came out just as loud as anything he ever said “SHE LOVES COOKING. SHE MEETS ALL MY STANDARDS. AND SHE IS A MEMBER OF THE ROYAL GUARD. HOW AWESOME IS THAT?”

“That’s cool Paps.” Sans was glad that grinning was his default facial expression. The way Undyne was watching him, made him feel very uneasy. Had she noticed the upside-down newspaper? He quickly put the paper down and edged over to stand between them and the couch, hoping Cody was staying put. “Uh… how about you show Undyne the kitchen?”

As they turned away to inspect the kitchen, Sans made sure he stood in the doorway, blocking off the view of the living room. From the corner of his eyes he could see a small silhouette peek up from behind the couch.

“I’m sorry Papyrus. This just won’t do! With this kitchen it’s no wonder your spaghetti turns into a loser-meal. You have to show those tomatoes who is boss! You have to beat them to a pulp! Devastate them with your cooking prowess. Make them quiver out of their peels at the merest glance of you!”

Sans was slightly pleased Undyne didn’t think their kitchen was up to scrap. If she would start cooking here, there was no telling what would happen. They might end up burning the house down!

Undyne had a solution handy. “How about you come visit me sometimes? We’ll use my kitchen. I’ll show you how to really wage war on vegetables!”

Sans risked a glance behind him and was just in time to see Cody reach the door. He gave the boy a quick thumbs up. Cody answered with a thumbs up of his own and then slipped out the door. After he was gone, Sans leaned back against the wall, letting out a sigh of relief. Glad that was over with.

In the kitchen Undyne and Papyrus continued their animated discussion. They would be at it for a while.

He allowed himself a fond smile. It seemed like they were genuinely having fun. How about that?  
  


***  
  


Sans was having quite a good day, all things considered. He’d managed to un-clean the house fairly fast. The human was on his way, and Papyrus had made a new friend. He figured a self-congratulatory drink at Grillby’s was in order.

He’d barely stepped out of the door, however, when he noticed two yellow eyes glimmer in the shadows. Their owner stepped out to meet him. 

“Hello Undyne.” he said calmly. “I thought you’d gone home already.”

“I’m on duty tonight. Tomorrow morning too. A royal guard’s job is never done.”

“Wow, sounds… busy. Aren’t you tired? You spent all day hanging out with my bro.”

“Tired? Me? Ha! I laugh in the face of…” her face twisted as she tried to stifle an unfortunately timed yawn. “...sleep”.

“I’ve never seen any royal guards down here besides the Dogs. Why the reinforcements?”

“There’s been a report that someone spotted a human down here.” Sans stiffened ever so slightly. “Oh really?” his grin widened. “Is this like the time those rabbit kids pranked the town with all those echo flowers?” That had been a fun prank, even if it had gotten a bit out of hand in the end. Fortunately no one had ever discovered Sans’ involvement.

Undyne grinned. “Yeah, it’s probably nothing. But that is no reason to be any less vigilant!”

“Well, good luck with that.” Sans raised a hand in farewell, then continued towards Grillby’s.

“If there is a human, it won’t take us long to find it.” Undyne called after him.

Sans halted. 

“Why would you think that?”

Undyne looked at him with an air of satisfaction. “There’s been an avalanche on the road out of town. It will take days to clear. No one is going anywhere.”

When it seemed like Sans wasn’t going to share her elation at the forces of nature conspiring to block passage out of town, Undyne gave him a searching look. Then she brightened.

“Ah, of course. How stupid of me.” She slapped her forehead with an audible thwack. “The royal scientist is your brother right? Don’t worry, doctor Gaster made it out of town just fine. He is being escorted home.”

“Great!” Sans managed. He wondered if Cody had been as lucky.

  
***  


By the time Sans left Grilby’s, it was quite late. The bar had been bustling with talk of the avalanche and the supposed human sighting. News traveled fast in Snowdin. Sans had stayed until it was just him and Grillby. He then helped the bartender clean up before going home. He liked Grillby. The fire elemental was quite bright (heh) and made for a good conversation partner.

The street was abandoned now. All the lights were out. Papyrus was probably sleeping already.

Something hit Sans on the back of his head. It was wet and cold. He spun around, arm raised, half ready to summon his battle-magic. Halfway between the library and the house he shared with his brother, stood Cody. He was grinning. Sans wiped the last of the snowball from his skull.

“Hey kid. Nice aim. You almost gave me skullfreeze.” He lowered his arm. “What are you still doing here?”

Cody pointed at the snow and shrugged. Sans caught his meaning. “Yeah, the avalanche. I heard. You’re one unlucky kid. There are royal guards all over town now, looking for you.”

It wasn’t his problem. And it was a terrible idea to get involved any more than he already was. “Welp, I’m going home. Good luck kiddo.” He began to walk.

Cody stepped forward, keeping pace. “I… I don’t know where to go.”

There it was. Sans could already feel his resolve weaken. If only this kid didn’t look so much like the other one…

“This is a really bad idea...But I just might know a way out of town…”

Cody brightened. “Really?”

“It’s kinda dangerous. But so is staying.” he shrugged. “Up to you, kid.”

“Please show me.”

Sans sighed. This was a REALLY bad idea.

  
***  


Cody gasped when they reached the ice. The lights from the ceiling made it glimmer and sparkle as if a nest of fireflies swam below it.

“Not many people use this route..” Sans explained. “...as the ice is usually too thin. But it has been freezing for days. It should be thick enough to get you out of town as long as you don’t do anything crazy. Best not to jump, dance or do somersaults. You know, just in case.” His smile became even broader. “Think of it as a shortcut.”

Cody’s eyes were still wide, as he stared at the lights. “Are those… stars?”

“Ceiling crystals. You can’t often see them here. But in waterfall, where you’re going, they’re very bright and always there.”

“Oh. I see.”

“Want to make a wish?” Sans asked, jokingly.

Cody looked confused. “A wish?”

“Isn’t that what you surface people do?”

“Oh…” Cody put a hand on his bag, as if whatever he was carrying in it gave him consolation. Something in his expression had changed. He had that serious look on his face that Sans spied on him whenever he thought Sans wasn’t watching.

“I only have one wish.”

He didn’t say more, instead stepping out onto the ice, carefully testing his weight. The ice held, yet Cody yelped in surprise as a snowball hit his cowboy hat. The hat fell in front of his face, held by a string that tied it underneath his chin.

Sans was laughing. “Welp, you kinda had that one coming. You could say Ice got you.”

Cody adjusted his hat. He grinned and scooped up a handful of snow from the ice. “That was really terrible!” he said, while flinging it at Sans, who dodged with ease.

“You’ll have to try harder than that kiddo. No one can hit me.”

“No? Then what was that earlier?”

“You caught me by surprise. That’s called cheating.”

Cody gathered more snow. “Are you saying you were skullible enough to fall for that?”

“Ha, good one kid.” Sans easily dodged another two snowballs. He scooped up an armful of snow and threw it at Cody, who tried to move aside. He managed to avoid getting hit, but he slipped and landed in the powdery snow.

Sans walked over to him. “You ok?” He pulled him up and helped him brush off. “I think that’s enough for now. It’s freezing and you don’t want to get snow all over you before you even reach waterfall. There’ll be enough opportunity to get soaked there.”

“Yeah… you’re right. I shouldn’t get distracted. I mean, thank you for your help. I appreciate it. I didn’t think anyone down here would… you know, be nice to me.”

Sans nodded. “I didn't think I'd spent my evening hanging out with a human either. But you seem all-right.”

“You seem all-right too.” he smirked. “For a monster.”

“Don’t underestimate us monsters.” Sans thought of Papyrus. “Some of us might surprise you.”

Cody didn’t look convinced.

“Goodbye Sans.”

“Seeya kiddo.”

As he watched Cody take off over the ice, Sans almost called after him. Where was the kid going to go? There was no way out of the Underground. The future didn’t look very bright for him. But what could he say? What could he do? They were all trapped here. The best he could hope for was to not get caught by the royal guard. To not end up in a laboratory. To not get killed. And who knows? Humans had strange powers. Powers which even Gaster didn’t entirely comprehend yet. Perhaps Cody could achieve something no one had ever thought possible.

His hopeful thoughts were interrupted by the sound of something crashing into the snow. Sans looked up to see a bright blue spear fly through the air. Cody was running, almost gliding over the ice. Pursuing him was Undyne! Her attacks were relentless. Cody dodged and jumped, but spears kept flying, sometimes changing direction in mid-air.

Cody jumped away, but slipped and his bag ripped free from his shoulder. It slid over the ice and took a while to come to a halt. The kid actually began to run back. Sans took a sharp intake of breath. What was he thinking?  
  
Another spear landed, this time right in front of Cody and a huge crack formed in the ice. The crack spread and shattered beneath the kid’s feet and he struggled to keep his balance. The next moment he had fallen into the freezing water, trying desperately to cling to the side and stay afloat. Another spear formed. Cody wouldn’t be able to dodge. Before Sans very well knew what he was doing, he summoned his own magic. A brightly glowing bone knocked the spear out of the way. Undyne swirled around. She looked shocked to see him.

“Sans?! What are you doing?”

“Don’t kill him, Undyne!”

Undyne’s eyes narrowed. “I thought there was something going on with you. But this? Think about what you’re doing. It’s a human, Sans!”

Sans calmly walked towards her. Not letting any of his inner turmoil show. His grin was plastered to his face, eyes shining. “Yes, a human. And all humans are to be brought to the royal scientist for experimentation. Or have you forgotten we need souls to break the barrier?”

Undyne looked a bit uncertain. “We can get the soul now… right?”

At the question in her voice, Sans knew he had her. He pulled a badge from deep inside his thick blue hoodie. He was very glad Gaster had made him keep it when he quit working at the lab. It was a bit faded, as it had been caught in Papyrus' laundry several times. Nothing escaped Papyrus’ laundry.

“As the royal scientist’s chief assistant, I am telling you; we need this one alive.”

Undyne did a double take as she looked at the badge. Cody, who was still clinging onto the edge of the ice, was looking at him too. The kid’s gaze was full of hurt and silent accusation. Certainly he understood Sans was bluffing to save his life, right?

“Fine.” Did Sans imagine it, or did Undyne actually look relieved? “She pulled Cody out of the water by the edge of his coat and roughly threw him onto the ice, as if he didn’t weigh anything. “You are under arrest.” she said. “Backup is on the way. I called them earlier. So don’t try anything.”

True to her word, they could see Dogamy and Dogaressa approach the lake now. Cody looked wet and cold and miserable and not about to start running or fighting. Sans could see his gaze flicker to his bag, which was still lying a small distance away. He took a step towards it, but Dogamy got there first and snatched it up.

“What’s this citizen doing here?” Dogaressa asked Undyne.

Sans answered before she could. “I was just leaving.” He looked at Undyne. "Be a pal and don't tell anyone I was here."

Undyne frowned, but nodded.

He could feel Cody’s gaze boring into his back, as he walked away. The way home had never seemed so long.

 

***

**Notes for the Chapter:**

> Welp, that could have gone better.  
> Notice any mistakes? Have any questions? Just want to throw me some random words? Feel free to leave a comment.  
> Tibia continued! Sorry (not sorry)


	3. The other side

***

Snowdin town had seldom seen such eventful days. Usually the news was as static and predictable as the frozen ground. But today exciting news kept streaming in. As if the avalanche hadn't been enough, a human had been captured! Right here in Snowdin! It was a royal guard by the name of Undyne who had managed to defeat the dangerous creature. She was a true hero! Surely there was a bright future ahead of her.

It took a day to clear the avalanche to the point that the guards could transport their dangerous prisoner back to the capital under heavy escort. Undyne went with them and the whole town watched them leave.

Papyrus was beaming when he came home.

"YOU SHOULD HAVE BEEN THERE SANS. IT WAS GLORIOUS. WE DIDN’T GET TO SEE THE HUMAN, BUT UNDYNE GAVE OUT AUTOGRAPHS TO ALL THE KIDS IN TOWN!”

He held up a piece of paper with a crudely drawn scribble on it. “AND TO ME” he added proudly. “DID YOU KNOW THAT THE KING IS GOING TO MAKE HER CAPTAIN OF THE GUARD?"

"That's great bro." Sans said absent-mindedly. He had refused to watch Undyne’s victorious departure from town. Instead he was lying on the couch, staring up at the ceiling. Papyrus gave him a concerned look.

"ARE YOU NOT FEELING WELL?"

"I um... It's ok Paps. It'll pass."

"DO I NEED TO GET YOU SOMETHING? DO I NEED TO MAKE SOME FOOD?"

Sans was seldom sick. Papyrus looked really worried now. Sans sat up straight and forced himself to sound a bit more cheerful. The last thing he wanted, was to upset Papyrus.

"I was just thinking... that's all."

"ABOUT WHAT?"

"Papyrus... am I being selfish?"

“PROBABLY.” Papyrus replied without missing a beat. Sans threw a pillow at him, which Papyrus caught with ease.

“I’m serious.”

“I THINK I NEED A BIT MORE INFORMATION.” Papyrus put down the pillow and sat down next to Sans.

“Why did you come to Snowdin instead of staying with Gaster in New Home? Why did you pick me?”

Papyrus was thoughtful for a moment. “BECAUSE… “ he said finally. “... GASTER LOOKED UNHAPPY. YOU DIDN’T.”

“Wait, what? Doesn’t that mean you should’ve stayed with him?”

“NO! YOU WERE UNHAPPY TOO. BUT YOU DIDN’T SHOW IT. YOU NEVER DO. AT SOME POINT YOU’LL WANT TO SHOW IT. AND I JUST FIGURED… YOU SHOULDN’T BE ALONE WHEN YOU DO.”

Sans swallowed. “You were thinking of me, but I never stopped to think about you. I brought you to this town, without even knowing if you want to be here.” Sans usually kept his cards pretty close to his chest. He wasn’t used to opening up like this. But yesterday’s events had put him in a melancholy mood. “To be honest I've been feeling like a pretty terrible brother lately." 

Papyrus brightened. "YOU'RE THE BEST BROTHER, SANS! WHEN YOU AREN'T MAKING THOSE AWFUL JOKES, BECAUSE THEN YOU’RE THE WORST. BUT THERE IS IN FACT SOMETHING I'VE BEEN MEANING TO TELL YOU."

This caught Sans' attention. "What is it?"

"REMEMBER HOW I NEVER REALLY KNEW WHAT I WANTED TO BE? I MEAN OBVIOUSLY SOMEONE AS TALENTED AS ME CAN BE ANYTHING HE WANTS, AND I HAVE BEEN SUCCESSFULLY PASSIONATE ABOUT MANY A PROJECT, BUT I NEVER QUITE FOUND ONE GOAL WORTHY OF PURSUING..."

Sans nodded affirmative. Papyrus had definitely been passionate about a lot of different things. Action figures, puzzles, spaghetti, squirrels...

"... WELL SANS, I NOW KNOW WHERE MY TRUE CALLING LIES! I AM GOING TO BE A ROYAL GUARD!"

"You.. what?" For the first time, Sans had no idea how to respond.

"I WILL CONVINCE UNDYNE TO TRAIN ME! IT'S GOING TO BE AMAZING. I TOO WILL BE A HERO ONE DAY. IT IS MY DREAM, NAY, MY QUEST!"

Sans had to smile at his brother's enthusiasm. "You'd be a pretty impressive hero, Pap."

Papyrus beamed "I'VE ACTUALLY GOT A QUEST!" he repeated, his voice high pitched with excitement. He hugged Sans then.  
  
Sans grinned. "Watch the bones, bro."

"UNTIL THEN, I WILL BE A SENTRY. SOUNDS COOL, DOESN'T IT? SINCE THE HUMAN SEEMS TO HAVE COME FROM SOMEWHERE NEARBY, SNOWDIN IS GOING TO NEED MORE PEOPLE WATCHING. THAT'S WHERE I COME IN. UNDYNE ALREADY OKAYED IT. SORTOFF. HEY, YOU CAN BE A SENTRY TOO. WE CAN SENTRY TOGETHER!"

"Heh, I dunno. Sounds like an awful lot of work." Papyrus frowned at that. "So what does a sentry do?"

"UH... SENTRY THINGS?" He raised a finger as an idea came to him. "PUZZLES! YES, THAT'S IT. WE SHALL DESIGN PUZZLES TO CONFOUND AND CONFUSE ANY HUMAN THAT COMES THROUGH."

Ah Papyrus. Sans loved how he could turn the most complicated problems into fun challenges. It was impossible to stay glum around him. "I think I'd like that." he said. "But first, there's something I have to do." He sighed "And I can't believe I'm doin’ it."

Papyrus eyes sparkled. "YES YOU DO!." he said.

“Do what?" Sans was starting to get confused.

"I KNOW YOU NEED TO GO TALK TO OUR BIG BROTHER. YOU'VE BEEN UPSET EVER SINCE HE VISITED. NO, YOU’VE BEEN UPSET EVER SINCE YOU STOPPED WORKING WITH HIM. YOU PRETEND LIKE EVERYTHING IS OKAY BETWEEN YOU TWO NOW, BUT I KNOW YOU’RE BOTH STILL ANGRY.”

Sans really had to stop underestimating his brother. "You could tell?"

"I'M YOUR BROTHER." Papyrus said with an air of confidence. "I CAN ALWAYS TELL. ALSO IT WAS REALLY, REALLY OBVIOUS.”

“It was?” Sans thought back to Gaster’s last visit. He grinned sheepishly. “Heh, I guess it was.”

Papyrus wasn’t done yet. “NOW I DON’T UNDERSTAND EXACTLY WHAT HAPPENED BETWEEN YOU. BECAUSE NEITHER OF YOU WILL TELL ME” he added accusingly. “BUT I KNOW YOU NEED TO WORK IT OUT." He paused, before adding. "AND THEN THE THREE OF US CAN BE TOGETHER AGAIN. AS A FAMILY!.”

“I uh… I’m not sure if Gaster needs family anymore.” Sans was sure Gaster wanted his lab assistant back. Anything more than that was pure speculation.

“OF COURSE HE DOES. DID YOU SEE HOW THIN HE LOOKED?.” So Papyrus had noticed too.

Sans gave him a fond smile. “Allright. I'll see what I can do. But I’m not doing it for Gaster. I’m doing it for you." _and for Cody_ .

***

It was the most awkward phone call ever. Sans hated every minute of it. Even though he wanted to try for Papyrus, the thought of making up with Gaster gave him no joy. If anything, he still felt anger! But maybe, just maybe, he could protect Cody. He’d never forgive himself if he didn’t try. If he let the same thing happen again. He shook his head, forcing himself to pay attention to the phone. He realized he’d drifted off and missed the last few sentences. Gaster had been monologuing again. For someone who didn’t like speaking out loud, he sure enjoyed the sound of his own voice.  
“I didn’t quite get that.” he said.

Gaster sounded a bit annoyed at his brother’s inattention. Yet there was an unmistakable glee when Sans had told him he was coming back.

“I will make arrangements for you. to stay in the lab.”

“Um… thanks, but no thanks.” If the lab was anything like he remembered, he didn’t want to be sleeping there.  
  
“You made the right choice coming back. I knew. it was only a matter. of time.”

“Yeah, well, don’t get your hopes up, I’m only coming back for a little while. One project. And there are…” He took a deep breath. Here went nothing: “...conditions.”

Sans could vividly imagine the frown that must be appearing on Gaster’s face right now. The royal scientist wasn’t used to anyone telling him what to do.  
  
“Conditions?”  
  
“You said you were working on a way to break the barrier, without needing to kill anyone.  
  
“That is what I said. more or less.”

More or less? Sans put a hand on his skull. Why was he doing this again? Gaster never told the whole truth. He couldn’t be trusted. Why was Sans allowing himself to be pulled back in?

“There’s a human coming your way.”

“I am. well aware.”

“If I work for you, we’re going to work on breaking the barrier without killing him. Without killing anyone. We’ll break the barrier and then we’ll sent the kid home. Everyone wins. Happy ending. Done.”  
  
His brother sounded amused now. “Such. sentimentality...”

“Skip the preaching. Do we have a deal?”

Sans could almost feel Gaster’s smile on the other side of the the phone line. “Done.”

  
***  


The trip to the capital was uneventful. Papyrus insisted on joining, even though Sans told him over and over again that he’d be fine alone.

“What about your sentry training? Are you going to disappoint Undyne already?” Sans couldn’t care less about the sentry thing, and he felt like a total jerk for pushing Papyrus like this. But he really would prefer to leave his younger brother out of this. The intrigue of the capital, the royal court, the lab… all these things were poisonous. He wouldn’t have gone near any of them himself, if he had the choice.

“I CAN TRAIN JUST AS WELL IN THE CAPITAL. EASIER, SINCE UNDYNE SPENDS A LOT OF TIME THERE. AND I’M ONLY COMING LONG ENOUGH TO GET YOU SETTLED.”

“Alright…” Sans conceded. “Only a day or two. Then you’re going back home.”

Sans got the both of them a room in a hotel outside of the city. It was expensive. Too expensive for them. Good thing Sans had decided Gaster was paying.  
Even outside the city, rumors of the politics and dealings of the capital and court were commonplace. There was talk that the queen had gone into voluntary exile, disagreeing with her husband’s newfound love for hunting humans. Sans had to smile at that. He didn’t know the queen, but it sounded like he’d like her.  
The room was spacious, with a huge bed, but barren. It lacked all personality. Until Papyrus opened his luggage and lined the walls with action figures and all his favorite posters.

“SEE? JUST LIKE HOME.”

“Any place you are feels like home, Paps.” Sans said fondly.

Despite his misgivings, he was happy his brother had come along.

  
***  


The lab looked much like he remembered it. Same tiled floors, grey walls. But there was a liveliness there, that hadn’t been there before. Sans supposed they had Gaster’s new assistant to thank for that. There weren’t many assistants who were able to work with Gaster for very long. Either he insulted them to the point of leaving, or he fired them for doing something that displeased him. Sans and Gaster had gone through their fair share of assistants back when they were still working on the Core.  
Yet Alphys and Gaster seemed to have come to some kind of understanding that worked for the both of them. At first Sans wasn’t sure what to make of the mumbling, awkward reptile. She looked like she’d rather hide behind her computer screen than shake his hand.

“Um… h...hello. You must be Sans. It’s n.. nice to meet you. I am Alphys.” She jumped as the machine she had been working on gave a small click and let forth a small puff of purple smoke.  
  
“Nice to meetcha too. Um… you might want to keep an ion that.” He winked.

“Yes… I should keep an eye.. ion? Oh, haha. I see.” Alphys let out a nervous giggle that was slightly too high pitched. It was a fairly good start, Sans decided. It’d be a relief to have someone else in the lab who was at least capable of laughter every once in awhile.

Behind him, Gaster cleared his throat. “If you are done with the. introductions. let me show you around. Much has changed. since you were last here.“

“Whatever you say, doc.” Sans followed Gaster through empty hallways and down a long elevator ride. The hum of the core got louder and louder. Gaster’s lab must be right on top of it.  
The room was meticulously clean. Even the few scattered notebooks were straightened and looked as if they were placed with deliberate care. A number of empty containers lined one of the walls. Soul containers, Sans realized. Commissioned by the king himself.

The hum of the Core was clearly audible here and Sans thought he could even see its glow coming from a large glass panel set into the floor.

“Are we…?” he began, impressed and slightly worried about the safety implications of being so close to the Underground’s largest reservoir of magic.

“Yes. We are right above the base of the Core. I am borrowing. some of it’s power. for my current experiment.”    
Sans whistled. Despite himself he was getting really curious to see what his brother was working on.  
“I will. show you.”

In a corner of the room stood a large device. It was about the size of a large mirror, or a table put on its side. Its surface was pitch black. Even the glow from the core did nothing to illuminate it. Looking at it made Sans feel dizzy, like he was falling. It gave him the creeps. A number of machines were hooked up to the frame, giving various readings that didn’t make much sense.

“That’s a pretty impressive coffee table.” Sans joked. As usual, Gasters face remained impassive at the attempted humor. Fair enough, it had been a pretty bad joke.

“It’s a window.”

“A window?”

Gaster smiled. “A dimensional. window.”

“Ohkay, you lost me.”

“King Asgore wishes to break the barrier. For that he needs. human souls.”

Sans made an impatient gesture with his hand. “Yeah, yeah, I know that part already. Feel free to skip the recap.”

“But what if. there is another method of breaking the barrier?”  
  
“Then that’s not much good if we don’t know about it. Honestly, I wouldn’t even know where to start.”

“Not in this timeline. but there are infinite possibilities. Infinite universes where we did find. another way. And through this. we can see them. We only have to find the right one. The right universe. The right time.”  

“The many worlds interpretation… I remember the theory.” He remembered many evenings, sitting with Gaster, theorizing, philosophizing, fantasizing… Back when things were good. Back when he, Papyrus and Gaster were inseparable. Back when he didn’t think anything could ever come between them.

“It’s not just a theory. I am making it. a reality. And you are going to help me. Observe.”  
Gaster flipped a switch and a loud buzz could be heard. A light above the window sprung to life. It was a small orb. No, Sans realized. No orb. It had the shape of a heart. Its cyan colour was so vivid that it flooded the entire room with its light.

Sans’ breath caught in his throat. He wanted to scream, yet when he spoke his voice sounded soft and hoarse.  
  
“The soul… you used the soul. You said… you promised...” He should have known. Gaster hadn’t changed. Everything was just the same as before.  
  
Gaster’s voice was firm as he observed the machine, his machine, come alive. “I told you the truth. I said that if successful. we would not need any MORE human souls. This one. we extracted earlier. as you well know. Your human is quite safe. And don’t worry. I’m being quite careful not to drain this one completely. The king. still believes he needs it.”  
  
“Half a truth at best.” Sans had managed to regain his outward composure, even though he didn’t feel the least bit calm.  “Do you have any more surprises for me?”   

“Only one.”

Gaster pressed another button and the window lit up.

 ***

Inside the window was another lab. It looked a lot like the one where they were currently standing. Gaster was there, using his sign language. “Only one.” he signed, right before pressing a button. Sans’ mirror image was nowhere to be seen, however. Talking to Gasters twin, was Papyrus. He looked annoyed with something. It was a strange look to see on his brother’s face. Something was off. Something in his eyes, the way he stood with his hands in the pockets of his coat, reminded Sans not of Papyrus, but of himself.

“What… what’s going on?” he stammered. He could see mirror Papyrus was saying something too, even if no sound came through.

“A different universe. We seem to have found one where your brother became the scientist. not you. A simple change of a few parameters. Switch a few numbers. and everything changes.” A ghost of a smile touched his face. “Dimensional travel is easy. once you work out the rules. Something small… One simple act of kindness. can be enough to tip the scales between one world and the next.”

The image faded and darkness overtook the window once more. It took Sans a moment to collect his thoughts.  
“You, you did it. You really did it.” he stammered. “All that theory… everything we talked about… you made it work.” He was impressed despite himself. Curious despite himself. For a moment he forgot all about Cody. Forgot all about his misgivings about Gaster. This was amazing.

“It doesn’t work as it should. I cannot hear them speak. I cannot control the images. They appear at random and only seldom. Not overly useful yet. We need to improve it.”

“Alright.” Sans nodded. “I can work with this. Show me what you did. We’ll go over it together.”  
  
“Together.” Gaster confirmed.

**Notes for the Chapter:**

> Oh no Sans, what did you do? You’re back on team Gaster! Is this a good thing, or a bad thing? What do you think?


	4. What would Papyrus do?

***

The next few days went by in a blur. Sans buried himself deep in Gaster’s calculations and notes. Papyrus dropped by the lab a few times to bring lunch. “YOU TWO WORK TOO HARD.” He told his brothers. Something Sans didn't often get accused of, so he made sure to cherish the moment.  
  
He couldn’t notice the glimmer of happiness in his Papyrus’ eyes without feeling a little happy himself. It was good, spending time together, all three of them. It felt right somehow. When Papyrus finally returned to Snowdin’ Sans was sorry to see him go.  
“Take care of yourself, Paps.” he said. “I’ll be back before you know it.” He hoped he wasn’t lying. This project was a lot more complicated, not to mention interesting, than he’d expected.  
  
Papyrus gave him a long, bone-crushing hug.  
“DONT WORRY SANS. I’M ALMOST A REAL SENTRY NOW. STEP ONE IN MY SENTRY-GUARD-HERO PLAN. I’LL KEEP SNOWDIN SAFE.”

“I don’t doubt it.”

Papyrus looked about ready to leave, but after a moment’s hesitation turned back to Sans, an unusually troubled look on his face.

“SANS?”

“Yeah?”

“I THINK THAT MAYBE I SHOULD HAVE STAYED WITH GASTER BACK THEN.”

Sans face fell. “What makes you say that?”

“I LIKE IT IN SNOWDIN WITH YOU, BUT GASTER HAS BEEN ALL ALONE HERE. I DONT THINK HE SHOULD EVER BE ALONE. BUT THEN I DON’T THINK YOU SHOULD BE ALONE EITHER. DON’T BE ALONE, OK?”

It was strange to see Papyrus so serious. “Ok bro, I won’t.”

  
That was easier said than done. The hotel room felt very empty after Papyrus had left, even though there were still some of his posters on the walls. Sans looked at them wistfully. They somehow made the Papyrus-shaped hole in the room even bigger. He started taking them down.

He was about halfway done when a piece of paper that was not a poster, sticking to the wall on Papyrus’ side of the bed, drew his attention. It was a drawing of him, Gaster and Papyrus, standing together and looking happy. They were waving. Sans took it, smiling as he studied it. Then he carefully folded the paper and put it in his pocket. It would be nice to have something of Papyrus with him in that gloomy place.

***

  
A week went by. It hadn’t taken the three scientists very long to fall into a convenient rhythm. Alphys usually worked by herself, upstairs. She wasn’t involved directly with the Dimensional Observer, as Gaster called the machine Sans had come to think of as "the window". Instead she worked upstairs, on the Core and more specifically the stability of the Core. There were some recurring bugs. One of which, she claimed, caused instabilities in the Underground’s weather patterns and might in fact be to blame for the recent avalanche in Snowdin.  
Sans occasionally helped her out. He had a fair deal of knowledge on the inner workings of the Core, having been his brother’s assistant back when Gaster designed it. In fact he’d been surprised Gaster was agreeing to let anyone work on his pet project with so little supervision. Gaster had always had a very low tolerance for lab assistants. But Alphys seemed unusually bright, not surprising, or she wouldn’t have landed this job. Maybe more importantly; Gaster’s current project took all of his time.

Most of Sans’ time was spent on the Dimensional Observer as well. Sans and Gaster came up with a way to make the window more dependable. They now kept the connection between the Soul, the Core and the window at a low, but constant, level, instead of activating it in short bursts as Gaster had done before. This provided them with a steady stream of images.  
It was a step forward, but it was also a bit disturbing. Figures appeared in the blackness, and faded again. Never anything lengthy or coherent, but enough to peak Sans’ interest. Sometimes they were happy, pleasant visions. Once he glimpsed a younger version of himself reading bedtime stories to Papyrus. But Sans occasionally caught glimpses of things he’d rather not see.

The barren hotel room he was still staying in didn’t help matters either. Ever since he started working on the window, his sleep had become fitful, interrupted with dreams fueled by the sometimes outlandish visions he glimpsed during the day.  
He usually saw himself. But these other Sanses were not him. Some came close, but others… One time he’d spotted a figure that instead of his normal bone white, was as black as the darkness in the mirror. The figure’s face flickered with distorted images and letters that made Sans nauseous when he tried to focus on them. But the worst part were his eyes. There was madness in those bloodshot orbs. Deep, unfathomable insanity. This was himself, gone wrong in the most horrible possible way. Sans had no clue what could have caused him to turn into this monster, but he was sure he didn’t want to find out.  
  
That night he dreamed he was trapped in a soundless, white void. Everything was far too bright, hurting his eyes. The air smelled of ozone and antiseptic. He woke up soaked in a cold sweat. He avoided looking into the window after that.

***

Gaster took the opposite approach. Sans often caught him staring at the window for much, much longer than Sans was comfortable with. Some evenings when Sans went home, Gaster was still there, sitting in front of the dark hole in reality that they had created. Sans highly suspected he slept there, in the lab. Gaster had always been pale and thin, but lately he looked even more worn than usual. He rebuked any attempt Alphys or Sans took to tell him to take a break. The last time Alphys had suggested it, he had reacted with such anger that Alphys hid in her office for the rest of the day.

When she still hadn’t shown herself, come evening, Sans decided to pay her a visit.

“Hey Alphys. It’s getting late. Time to go home. What do you do for fun around here?” He needed a break. He really, really needed a break. Words could not express how much he missed Grillby’s.

Alphys jumped up from the book she’d been reading. Sans could make out the cover: Higher mathematics and their practical applications. The book fell to the floor. Alphys scuttled after it.    
“Uh… oh, hi Sans. I… um… didn’t hear you come in.”

“You ok? When the doc shouted at you, I thought you were going to faint or something.”

Alphys quickly collected the book, placing it on her desk. “Oh.. thank you. I’m fine though. Ha...ha, you wouldn’t think he could make so… much…. noice, would you? He’s usually so quiet.” she trailed off.

Sans bent down, picking up something that had fallen from the book. It was a comic. Alphys turned bright red.  
“Samurai Robot Ghost C” Sans read aloud.

Alphys snatched it from his hands. “That’s mine!” She squeeked. The loudest sound he’d heard from her yet. “It’s… research.”

Sans shrugged. “Looks fun. Like something my brother would read.”

“I don’t think doctor Gaster would…. Oh uh… your _other_ brother. Of course.” She sighed. “Look, don’t tell him ok? I just need to unwind sometimes. This job is...h...hectic.”

“Your secret’s safe with me.” Sans grinned broadly. “If you let me read it.”

Alphys’ entire composure changed. “You’re interested in Samurai Robot Ghost C?” she asked incredulously. “It’s really good. He’s a samurai robot, but also a ghost. On a mission to avenge his master… The anime is even better! Do you… um… would you like to see it?”

After days of work stress, poor sleep and disturbing visions, there was something disarmingly pleasant about hearing Alphys talk about her cartoons.  
  
“Sure." he shrugged, effectively hiding how happy he was to have her company. "I have nothing better to do.”

 ***

 After that, Sans spent most of his evenings hanging out with Alphys in her apartment, watching one of the many anime series she so avidly collected. It was strangely relaxing and just what he needed to wind down after another exhausting day in the lab. He found he really liked the somewhat eccentric lizard. And it wasn’t like he and Gaster were showcases for normality. In fact, Gaster seemed to sink deeper and deeper into whatever it was that was happening to him. The otherwise meticulously ordered lab was becoming more and more of a mess, notes spread out haphazardly.  
Sans noticed one piece of paper unceremoniously taped to the wall, saying “DETERMINATION” in capital letters, with several lines under it for emphasis. He frowned. Gaster really needed to get out more.  
Sans was having a harder and harder time to talk to him. Often Gaster was too distracted to even respond. He wished Papyrus was here. Papyrus always had a stabilizing influence on Gaster. Unfortunately it would still be another two weeks for their brother to come visit again.

  
Now that his brother’s attention was elsewhere, Sans did have an easier time moving through the lab unobserved. One day he decided to visit Cody. He brought a stack of books, candy bars and soft drinks that he’d pilfered from Alphys’ personal supply. He knew Gaster kept the kid locked up in another part of the lab, close to his personal quarters. He didn’t go down there much, letting Alphys handle most of the human-caretaking. As Sans walked past Gaster’s personal quarters he knocked on the door. No one was in. Of course not. The royal scientist hadn’t used his quarters in quite some time. A few doors further was a cell, reserved for the “specimen”. Sans was pleased to see that his badge still had clearance for it.

The room inside was divided in two by a blue force field. On Cody’s side there was a bunkbed, a bucket and some food; cup noodles from the look of it. Figures. Alphys was in charge of bringing food to the human. He’d probably been living off nacho’s and noodles for the past week. Cody was lying on the bed, hat covering most of his face. He looked very small.

“Hey kid.” Sans said, waving a hand. Seeing how miserable this little cell was, the skeleton immediately felt bad for not visiting earlier.

Cody instantly sat up, startled by the unexpected voice. When he saw who it was, he merely stared. His gaze went right through Sans. And there was something there that Sans had never seen on this human’s face before. Fear. Cody was afraid of him.

“Um… look. I’m sorry I didn’t visit before. I just wanted to say, you’ve got nothing to worry about. I’m gonna get you out of here.” Sans felt like the worst monster in the underground. How could he have left Cody all alone here? He’d been so occupied by his work and… but no, that wasn’t the whole truth. He’d been wanting to put off this confrontation. Scared of being judged for his part in this whole situation. Scared of being reminded of that other human, trapped in this same cell. The one whose soul was now powering the machine he and Gaster were working on…

“Why should I trust you?” Cody asked. His voice was shaking. “You’re one of them. You’re working with _him_.”

“I know it looks bad. But trust me, I’m looking for a way to save you. We’re working on a machine that will help. We will find a way to set monsterkind free, without anyone getting hurt. And then you can go home.”

“Set… monsterkind… free?” The kid looked horrified. “Unleash all of them?”

“Relax. I told you, no one is going to hurt you…”

Cody rolled up a sleeve of his coat. The arm underneath was red and blue with nasty bruises and something that looked like puncture marks.

Sans eyes widened. _He didn’t…._

“Did Gaster do that?” he already knew the answer yet dreaded hearing it all the same.  
  
Cody shuddered. “He didn’t tell me his name. He was tall. Much taller than you. I think he’s your boss?”

The light in Sans eyes flickered, then died, going entirely dark. “ _This_ _ends_ _now_.”

***

Gaster was sitting on a chair in front of the window, staring into it. Occasionally he wrote something down in one of his many notebooks. A small pile was starting to form around his feet. He gave no sign that he noticed anything when the door opened.

“We need to talk.” Sans could barely keep the anger from his voice.

At first Gaster didn’t respond. But then, slowly he turned around. His eyes were distant.

“Yes?” he signed.

“Damnit Gaster, I need you to listen to me.” Sans spat. The use of his name did seem to reach Gaster as his eyes finally focused on him and he spoke his next few words aloud.

“I’m here. My apologies. brother. I was lost. in thought.”

“What did you do to Cody?”

“Who?” Gaster looked genuinely confused.

“The human!”

“You gave it a name.” His brother emitted a low chuckle. It sounded like rocks scraping together.

“Not funny. And coming from me, that means a lot. Whatever you’re doing to him, stop doing it. We had a deal.”

“If I recall correctly. and I do. our agreement was that I would not kill him. and sent him home. after we break the barrier. It is not my fault that you are not. more specific in your wishes.”

For once, Sans wished they had worked out how to get the Gaster blaster to work, so that he could show his brother the literal meaning of the name. He clenched his fists.

“It stops. Or I’m out of here.”

Gaster raised his hands in a gesture of peace. “Calm yourself brother. I have what I wanted. I will leave your human. alone.” He reached for a drawer and produced a small flask. Inside was what couldn’t have been more than a drop or two of a fiery red liquid. “Determination.” Gaster said, as if that explained anything. He stared at the flask like cat would at a mouse; an utter fascination and joy at the prospect of learning all its secrets.  
“The humans have it. in such an exquisite form.” When Sans didn’t respond he added: “It’s a side project. of sorts.”

Sans glared, torn between childishly lashing out and turning his back on his brother altogether. His thoughts were racing. He should be out of here. He should leave. He was in over his head. Any one of Gaster's projects was enough to drive a normal monster insane. And here Gaster was working on several. As usual, he didn’t even know the half of what his brother was up to. He should leave. He really should. But he couldn’t. He made a promise. Not to Gaster; but to Cody.  
He couldn’t save them all. He couldn’t save the previous one either. But if he could save one human, it might just be worth it.

He felt himself shaking with frustration.

“Sans.” Gaster said, and his voice was almost kind as he reached out and placed a boney hand on his younger brother’s shoulder. “It’s raining up there.”

“What?” Sans wanted to shake himself loose, but something in Gaster’s tone gave him pause.

“Nothing compares to the sounds of a thunderstorm.  The feeling of rain. The sun. warming your face. And there are stars. Not like the crystals. The stars move because there is so. much. more. out there. Every star is. a sun.” He wasn’t looking at Sans. Gaster’s face was turned upwards, as if he could see the stars and feel the rain, even through the many layers of concrete, metal and earth that separated them from the surface.

“You may think me. cruel. but it is nothing compared to the cruelties inflicted by the humans. upon our people. upon us.  
Myself. the king. even you to an extent… We became what we are because of them. For better. or worse.”

He was quiet for a moment. Lost in thoughts Sans had never associated with his brother. Sans felt his anger not vanish, but retreat, to simmer in the back of his mind. He noticed his hands were balled into fists. He slowly relaxed them.

"It ends." He stated again. "Leave the kid alone. Alphys and me will take care of him."

Gaster nodded. "You have. my word. Satisfied?"

"Do I have a choice?"

"Always." With that he turned back to his notes.  
  
Sans watched him work, his white lab coat contrasted against the black window. He should leave. He really should. But he couldn’t. He was caught in Gaster’s web, as much as Cody, as much as Alphys. And now he would see things through till the end.

***

More days passed. More research. More evenings spent watching anime with Alphys. More visits to Cody, who, now that Gaster no longer came to see him, was looking a bit more at ease with Sans’ presence as well. The kid even cracked a joke every now and then. Sans brought him comic books, science books, everything he could get his hands on. Alphys was too shy to talk to Cody herself, but she loved hearing Sans talk about him. She had a great fascination for humans. She even helped Sans pick out comic books she thought the kid would like.

They were making progress on the window too. Sans had discovered a way to tweak the machine that made the images more stable. And he was close to finding a way to switch them around, almost like a television. He had already managed to get it to work several times. A way to break the barrier might be just one channel change away.

Unfortunately every tweak he made to the machine, also seemed to make the window more fragile. This was a cause for concern, seeing how it was hooked up directly to the Core, the largest magical power source in the underground.  Fortunately neither Sans or Gaster were quickly unnerved by a little danger.

Sans was more worried about his brother's mental well-being than about potential magical explosions. Fortunately it would only be a few more days till Papyrus came to visit, and they would no doubt be able to persuade Gaster to take a long break. Get out of the lab for a while. Maybe visit the city.

All in all, things were perhaps not going great, but they were ok. Sans had the feeling that a breakthrough was right around the corner.

 ***

Two days before Papyrus’ visit, the letter that changed everything arrived.

It was a very fancy looking envelope, all gold letters and ivory seals, addressed to the royal scientist. Since it was very hard to get Gaster away from his spot at the window, Sans had taken it upon himself to open his brother’s mail. He hesitated when he spotted the royal seal, but not for very long. The handwriting inside was as fancy as the envelope, with long loops and curls at the end of the words. It read:

  
_“Dear Doctor Gaster,_

_I hope this letter finds you well. It has been some time since we sat together in my garden, enjoying a pleasant cup of tea. I’m sure you remember what we spoke about back then. And your results were more than I could hope for. Your method of capturing human souls in the “soul jars” will be the key I need to set us all free. I am forever in your debt and will make sure your name resounds throughout monster history._

_But alas, I write this letter with less pleasant business in mind. It has come to my attention that a human was captured recently and brought to your laboratory to be detained there. Now that we have perfected the soul containment method, I feel that I can no longer delay that which needs to be done, nor do I want to cause prolonged suffering to the humans who will be our salvation. I’m sure you will understand, dear friend, that I cannot place this burden on anyone except myself. I have arranged for a transport to retrieve the human and bring them to my castle. It should reach you a day or two after this letter. Please arrange for a few of the soul jars to be brought to me as well._

_Together we can put an end to these dark times._

_Your friend,  
_ _King Asgore”_

 

They were out of time.

What could he do? Take the letter to Gaster? Pretend he hadn’t seen it? A single question made its way towards the front of his mind, drowning out all others.

_What would Papyrus do?_

Sans crumbled the piece of paper, royal seal and all, and made sure to burn it in one of the lab’s gas burners.

There was only one answer.

**Notes for the Chapter:**

> One of the rules I have set for this story is that it's a tale of three brothers, not two, even if Sans and Gaster get most of the spotlight. And so Papyrus will play a role in every chapter. Even if he's not always physically there. Even if it's just thinking of him that steers Sans into a certain direction. I see Sans very much as the middle brother. Caught in the center. Forever pulled between the two extremes that are Gaster and Papyrus. 
> 
> So what would Papyrus do? What will Sans do? Tune in next time to learn the answer. And look at me, managing to sneak an Error!Sans reference in there! (Cause who doesn't love Error?)


	5. The truth

***

The elevator ride seemed to take forever, inviting both boredom and contemplation. Neither did anything to change Sans’ mind. He was going to free Cody. Consequences be damned.  
  
_*ping*_

The door to the elevator opened and the Sans’ grin became very forced indeed when Gaster walked in and pressed the button that would take him to his lab. Sans had assumed Gaster was already in the lab, like usual. Why did his brother have to pick today, of all days, to go for a stroll? It was an impressive feat of self-control that kept him from swearing out-loud.  
  
The two skeletons stared ahead as the elevator resumed its long journey, silence only broken by the regular hum-hum-hum of the elevator.

“What. are you up to?” Gaster asked eventually, with the mild interest of someone who has nothing better to do.

“Oh, you know.” Sans hoped he was sounding smooth and casual. “Doin’ some... stuff. You?”

“Also doing. stuff.”

“Oh.”

The elevator ride resumed.  
There was more silence.  
  
The awkwardness became close to unbearable until Sans couldn’t take it anymore. It didn’t used to be this way. He, Gaster and Papyrus… they got along. Being together with his brothers used to be the best feeling in the world. They do anything as long as they were together. And he knew they had felt the same.

“Hey Gas, when did we stop talking?” He asked softly.

“We are talking.”

“That’s not what I meant.” He groaned. “You’re such a bonehead sometimes.”

A wrinkle that might have been a frown formed in Gaster’s usually smooth forehead. He turned to a control panel in the wall and entered a short code. The elevator ground to a halt.

This was not good. Sans had rescue plans to get to. “Um… What’re you doing?”

“Let’s. talk.”

“Look doc, this might not be the right time…”

Gaster gave a dismissive wave of his hand. “It never is. the right time.  And you were right. We should. discuss this. You are angry. because you blame me. for the death of that human.”

“Blame you? Of course I blame you! You killed her, right in front of me!”

“And what. were you doing?”

Sans was beginning to sweat. Must be the poor air circulation in the elevator. “I wasn’t doing anything! It was all you.”

Gaster raised his arms, point made. “You did not do. anything.” He bent over, so he could look Sans into the eye. “You cannot overcome your anger. because it is not me you are angry with.”

“I…” Sans was looking for the manipulation behind Gaster’s words. The trick. There had to be one. It’s what Gaster did… He twisted the truth around to suit his needs….

Gaster wasn’t finished. His tone was soft and not at all unfriendly, even if his words stung like needles. “Passivity is in your nature. It’s where you I and Papyrus are different.”

“Don’t you dare drag Papyrus into this!”

“I act. Even if my actions are not. what you’d call “moral”. I do what is needed. That. is my nature." Gaster was in full lecture mode now. “Papyrus reacts. He does not. accept any perceived injustice. He sees the good. in every being. and strives to bring it out. That is his nature. You, Sans. you observe. you wait. you do what is required. if you must. and only then. That is your nature.”

Sans was not about to let Gaster rattle him. "Heh, you got me. I am terribly lazy. But I still do whatever it takes.” He had acted plenty of times. He saved Cody back on the ice. He’d do it again… because… because… _that’s what Papyrus would do_ , a treacherous voice in the back of his mind whispered. He tuned the voice out. What if it was? Was that so bad? It was still Sans, making the choice. It was still him. “All this “nature” stuff is no excuse. You’re just telling yourself a nice fairytale to help you sleep at night. Pretending you’re the one who has to “act”, even if acting means killing kids.” Sans looked him up and down. “Tho from the look of you, you haven’t been sleeping much. Is it all that research you’ve been doing? Or is my brother, the great royal scientist, finally developing a conscience?”

Gaster sighed. His next words were so soft, that Sans had to strain to hear them.  
“You and Papyrus. Everything I’ve done. everything I am about to do. I do for the two of you. I want to give you rain. I want you to smell. grass and feel. sunshine. I will give you. the stars.”

Sans scoffed. “And you called me sentimental.”

“Someone. has to play the villain. There is no gain. without sacrifice. I do not. regret what I did. What I will do. But… I do regret what it did and will do. to you. and by extension to Papyrus.” Sans hadn’t been wrong earlier. Gaster looked very tired and for just an instant, very sad. “Perhaps it would have been better. for you. to have stayed away from me after all. Unfortunately. it is too late now.”

“I used to think that too. But before he left, Papyrus said something... He said you should never be alone. Perhaps he was right. Sometimes I think Pap is a lot smarter than either of us boneheads.” Despite the harsh words they’d exchanged, Sans felt something he hadn’t felt in a long time: A smidgen of sympathy for Gaster. Perhaps it was not too late. Perhaps things could still be fixed between them. After Cody was gone, they'd have all the time in the world to work things out. Assuming Gaster would forgive him for what he was about to do next...

As if he was guessing his thoughts, Gaster looked at him for what seemed the longest time, his expression unreadable. Just as Sans began to feel uncomfortable, Gaster pressed a button and the elevator continued moving.

Sans leaned against the wall, trying to look casual even if he felt anything but.  
“For what it’s worth. Papyrus is going to join the Royal guard. He wants to hunt humans.” How’s that for your reactive, seeing-the-good-in-all-creatures theory?”

Gaster’s lip twitched into a semblance of a smile. “We both know. that he’ll fall in love with a human. before he hurts one.”

Sans chuckled despite himself. “Gas! Did you just make a joke?”

The smile was gone, but there was a sparkle in Gaster’s eyes that hadn’t been there before. “I don’t know. Did I?”

As if he’d timed it that way on purpose, the elevator chose that moment to arrive at the lab. Gaster stepped out, leaving Sans to pursue his plans.

***

Cody was asleep when Sans arrived in the cell. The kid was wearing a pyjama Alphys had bought for him in New Home. It had a picture of a cartoon crab on it.

“Hey kid, what did the gardener say to the tree?” Sans said, as he stepped through the door. He used his badge to deactivate the forcefield.

Cody sat up, rubbing sleep from his eyes. “Sans? What?”

Sans picked up the pile of Cody’s clothes that were strewn on the floor and tossed them at him. “It’s time to take your leaf.”

Cody’s eyes were huge. “You’re busting me out?

“Heh, well kids like you shouldn’t be cooped up inside on nice days like this. Now go on, get dressed.”

He didn’t have to say that twice. Cody quickly put on his clothes, his coat, his scarf and Sans finished by putting the hat on the kid’s head.  
“Perfect. Now follow me.”

Cody made to follow, but hesitated for a brief moment. “You aren’t going to get in trouble for this, are you?”

“Nah.” Sans lied.

***

The hallway was dark and quiet. Just the way Sans wanted it. Yet there was an eerie quality about it too. Like a school after closing time. A place no one was supposed to be.

Instead of following, Cody was lingering behind.

“What’s up?” Sans asked finally.

The response was barely more than a whisper. “My bag… I need my bag.”  

“Right.” The kid had, had a bag with him. Sans had nearly forgotten. “It’s probably in Gaster’s quarters. He keeps all human items there. You know, in case he feels like doing some late night studying. Follow me.” He turned on his heels and used his badge to unlock the door they had just passed. “Ha! Perfect score.” he said with a wink at Cody, as the door opened.

As Sans expected, the room inside was barren. Cody’s cell had almost been more cheerful. There was a bed, but it didn’t look like it had been slept in in some time. There were a few research notes on Gaster’s desk, but they were old, serving as further proof that his brother spent his nights down in the lab these days. The room smelled stale from a lack of air circulation.

They found the bag in a dusty cupboard. It hadn't been touched in a while. Cody was drawn to it like a pin to a magnet. He grabbed the bag and held it close.

His movement stirred something else in the cupboard: A ribbon floated down to the ground, drawing lazy circles in the air.

Sans stared at it.

Cody stared at it.

No one spoke.

The kid was the first to move. He gently picked up the ribbon and held it close to his face. 

This was really not the time. “Let's go kid.”  
Sans began to move, but Cody held his ground. His eyes were downcast, obscured by his hat.

“I never told you why I climbed up to mount Ebott, did I?”  

A feeling of dread crawled up Sans’ spine and settled there.  
He stopped walking, didn’t speak, couldn’t do anything but listen to the horrible words that kept coming from the kid’s mouth. This was it. This was his punishment. Karma finally caught up with him.

“When my sister vanished, I knew where she’d gone. I knew I had to find her. It was my fault she left. If I hadn’t shouted at her…  
Our parents were so sad, so scared, they paid no attention to me. They had no clue what I was about to do. But I’m not a little kid anymore. I prepared. My dad never realized that I knew where he kept it...”

Cody opened the bag and there was a glint of metal as he turned the weapon over in his hands.  
There was a weight to the gun. A heaviness that pulled Sans gaze, every bit of attention he could muster, towards it. It was impossible to look away. A human weapon. More real than any weapon made by monsters. And infinitely more dangerous to them.

“She’s dead, isn’t she?” Cody stated matter-of-fact. “I know. I’ve known for a while. We were always close. My mom said it was a twin-thing. If she was alive, I would know it. And I knew it when she died.”

Sans felt frozen, paralyzed. He forced himself to speak. “Listen kiddo….”

“Don’t “kiddo” me. No more nicknames. No jokes. Just tell me what happened..” Cody’s voice shook with anger, his eyes brimming with tears. An undercurrent that threatened to overflow. “It was him; your boss. Wasn’t it?”

In his mind’s eye Sans saw the girl, tied to an operating table. She was crying. Pleading for help that never came. She was so small. So helpless. Gaster loomed over her, stretched impossibly large over the tiny form below him as he reached out to her..

“Gaster was the one to do it, but it might as well have been me.” Sans said slowly. “I did nothing to save her. Heh, I guess I’ve always taken the easy way out. That’s just what guys like me do.” He was prepared for whatever was coming. Prepared for the fury he so rightly deserved. Prepared for the feel of the cold gun against his skull. And he was still grinning. Couldn’t stop. It was all too perfect. Too horribly, sickeningly perfect.

What he was wasn’t prepared for was what Cody’s said next:  
“That’s not true. You tried to save me. You didn’t have to do that. You’re the only one who has been nice to me.” There was a desperation to his voice. “My only friend down here. You’re not like _him_. You’re a good person.”

“Good? Maybe once upon a time, kid.” Sans hadn’t felt like he was “good” in a long time. Too much had happened. Gaster had happened. And he really couldn’t blame it all on Gaster. That was just a convenient excuse. His brother had been right about that.  

Cody lowered his gun, letting his shoulders slump. All anger seemed to leave him until all that was left was a very small, very broken kid. “I don’t believe you hurt her.” He wiped his eyes with his sleeves. When he looked back up at Sans, his face was full of resolve. “And I won’t hurt you either.”

Sans wanted to say something, but didn’t find the words. He sat down on the floor with his back to the wall. He felt exhausted. Cody dropped down next to him, cradling the gun.

“What are mom and dad going to do if I don’t come back either?” he still sounded close to tears, even if his eyes were dry.

“That's not gonna happen. I’ll get you out of here.” 

“There is no way out of the Underground.” The kid hugged himself. 

“What, you’re going to let a little thing like the barrier stop you? Lucky for you, I’ve been making progress on the machine down there. We’ll figure out a way to break the barrier soon. All you need to do is hide for a while.” Perhaps he could sent Cody to Papyrus. If he explained the situation, certainly Paps would understand. Or the kid could hang out in Waterfall for a while. There were plenty of hiding spots there. Sans could think of several of the top of his head.

Cody sprang to his feet as if stung. “You can’t do that! If you break the barrier, you’ll set all the monsters free!”

"Yeah, that's the..." Sans started, when a sudden, loud, scream drowned out the rest of his words. It was high pitched, like fingernails going over a chalkboard. Cody put both hands over his ears in reflex. Sans shivered. _Gaster_. He had never heard his brother scream like that, had never heard so much agony in any voice. It was terrifying.

He got up and took Cody by the shoulders. “Get out of here. No, don’t argue. Second hallway on your right, take the elevator. It can take you all the way out. Code 19-20-1-18-19. Got it?”

“I… “

“Repeat it.”

“19-20-1-18-19” I... I‘ve got it.

Sans didn’t wait to see him leave. He ran for the door. Somewhere below him, Gaster was still screaming.

***

The screaming stopped when Sans was halfway down to the lab. This did nothing to reassure him. If anything it made him more anxious. Gaster might be a cold-hearted egomaniac, he was also his and Papyrus’ brother. He was _their_ cold-hearted egomaniac. And that made him one of the two most important people in Sans’ world.

Slow, slow, far too slow he managed to get to the lab and entered. Gaster was lying on the floor, eyes blank, mouth open in a silent scream, body racked by spasms.

“Doc, Gas! What happened!” Something was wrong with Gaster’s right eye. It drooped down, as if his face was made of wax and had gotten too close to a candle. Sans took him by the shoulders and looked at him. “What did you DO?”

From a corner of his eye he could see something catch the light. The flask that had held the Determination lay broken on the floor, no sign of its contents. And now Sans could see a syringe near Gaster’s right hand, leaving no doubt as to what had happened. Gaster had injected himself with Determination.  
“Why?”  
  
A light went on in Gaster’s good eye. Instead of its normal white, it was red. Despite the great effort it seemed to take him, he focused on Sans. The spasms were fading. The sight of his brother seemed to calm him..  
“Because it was. necessary.” His voice was steady, even if it was hoarse from shouting.

“What, why? You of all people should know how dangerous…”

“Listen… Sans. there isn’t much time. Do you see? Do you see the soul reacting to a new. presence?” He raised a shaky arm to point at the machine. Its cyan glow was pulsing. Waves of blue flooding the room and casting Gaster in an eerie light.  
Gaster chuckled hoarsely. “All it took. was a little patience.”

Sans didn’t understand what he was talking about. What was the machine reacting to? All he could think about was Cody. Did the soul react to him freeing its twin? Something Gaster had said, what seemed like forever ago, came back to him: _One simple act of kindness, can be enough to tip the scales between one world and the next._

The inky black surface of the window seemed to ripple with every pulse. Like a liquid that was being disturbed by unrelenting approaching footsteps.

“This…” Sans started. “Gas…”  
  
Gaster ignored him. Too caught up in his own thoughts. “Did you think I was content. to use the window to merely watch, brother? To observe these other realities? No. we can go there. We can visit. other worlds. And more than that. So. much. more.”

Sans helped his brother to sit up. Gaster leaned heavily on his arm. “More?” Something occurred to him. “You told me this was about finding a way to breaking the barrier. Was that even true? Was anything you told me true?”

“You always did think. too small. With enough Determination. we can change the world. Go back. retry. Again. and again. Until everything is. just. the. way. we want it. Breaking the barrier is just the start. I have seen such. possibilities.”

There was something bright, almost feverish in his red eyes.

“You’re not thinking clearly.” Sans wanted to recoil from what he saw there, but he couldn’t without letting his brother fall, so he suppressed the urge and continued to support Gaster instead.

“Quite contrary. Everything is. so. very. clear. now.”

Something in the window caught Gaster’s eye and he doubled over as a new spasm went through him. When he looked back at Sans, there was sadness in his voice. “Ah, but I am afraid... we have run out of time. It is a shame. We could. have changed. the world. You and I.”

“What are you talking about? We need to get help. Get you to...”

“He approaches.”

When Sans looked back, Cody was standing in the doorway. Gun gleaming in the pale blue glow of the machine.

His eyes burned with determination as he aimed the weapon straight at Gaster’s heart.

***

**Notes for the Chapter:**

> Oh yeah! Shit just got REAL. Tune in next time, as several characters in this story are about to have a very bad time indeed. *insert evil laughter*  
> If any of you made it this far into the story, I'd love to hear your thoughts!


	6. Justice

   
The cyan glow in the room intensified as Cody stood there, gun aimed at Gaster, face impassive. The light made him look otherworldly, like a vengeful spirit. The only sound was the steady hum of the Core. But even that sounded anxious, almost hungry.

Sans was the first to speak. He stepped in front of Gaster, letting go of his brother’s arm. “Hey come on kiddo, let’s talk about this. You don’t want to… ” He yelped, as he was swept off his feet and shoved against the far wall by an unseen force. Gaster slowly stood, one arm raised as he held his brother in the grasp of his magic.

“I’m sorry Sans.” He said. Despite just having pushed his brother out of the line of fire, his eyes remained locked on Cody. “But this is between me. and the child.”

Sans struggled against the force that was holding him pressed against the wall, but he was unable to move. Gaster had always been better at magic than him, even though the arm he was using to keep his spell in place was shaking.  
  
Cody took a step forward. “Tell me one thing.” He said icily.  
Another step. “Did you even know her name?”

Gaster returned his gaze, expression unreadable. “I do not. But I know yours.  It took me a while to remember. But she was convinced you’d come. She was calling for you to come help her. until the end.”

Hatred flared in Cody’s eyes. “You have no idea how this feels. You have no idea what it’s like to lose a sister.” His finger twitched on the trigger.  
  
Sans fought with all his might against the magic that held him.  
“Kid… Cody, don’t! Please! Don’t kill my brother!”

Cody’s eyes widened and he actually met Sans’ gaze for a second. “Your… brother?” His arm was shaking now. Tears stung in the kid’s eyes, blurring his vision. The next few seconds seemed to happen in slow motion. Cody’s shaking hands pulled the trigger. The bullet cleaved the air. But it didn’t hit Gaster. It sailed past the scientist and lodged itself into the Window. The device gave one last blue pulse and then it exploded, pitch black shards shattering outward at deathly velocity.

Sans felt something hit his eye and lodge in the back of his skull. It _hurt_ , the worst pain he had ever felt. He grasped his head with both hands, screaming. The world seemed to explode. There were symbols, numbers, words, _code_ , everywhere. It flashed through his mind with every sound, every smell, every sight.

Through his pain-filled haze, Sans could see that part of the wall that had shielded them from the Core had been blasted by the explosion, as had part of the floor. The Core was right beneath them, spewing orange fire and roaring in sync with the waves of pain that wouldn’t stop assailing his head.

A figure was lying nearby. Gaster? He couldn’t really tell. Sans forced himself to crawl over to the shape. It was Cody. His clothes were soaked by a red fluid. Sans knew enough about human biology to recognize it as blood. Dozens of tiny shards had pierced the human’s body, but the worst was a much larger shard that had gotten stuck in Cody's chest. He was barely conscious, barely alive. The blood was starting to form a growing pool on the floor.  
  
When Sans looked at him, he could see letters flicker in front of his left eye: "HP: 3"  
  
“Kid?” Sans barely recognized his own voice.

“Hey Sans…” Cody said hoarsely. He sounded very faint, very far away. Sans had to strain to hear him over the roar of the Core.  
  
HP: 2  
  
“Just…” Sans trailed off, unsure of what he had been about to say. Just what? The kid was dying. Nothing he said could change that.

“...I guess I messed up, huh?” Cody laughed, a rasping sound that immediately turned into a coughing fit. “In the end I couldn’t… couldn’t shoot your brother. Didn't want to do that to you...”

The Core flared up and with it Sans felt another wave of pain shoot through his head. He took Cody’s small hand into his own. 

HP: 1

“Hey... Sans?”

“Yeah kid?”

“What did… the gardener say to the tree?”

Sans felt tears sting his eyes. “Time to take your leaf.” He wiped the tears away and when the world came back into focus, Cody was dead.

HP: 0

He let himself slump down next to the small body. His head felt as if it was about to explode. Staying conscious was so much effort. Too much effort. The Core flared again, heat and energy washing over the room. The black shards reacted to it. Some of them turned liquid, melting into the floor. Into Cody’s lifeless body. Into Sans’ skull.

He could no longer keep it back. His mind was assaulted by images. There were many, too many to comprehend. There was him. In endless possibilities. Endless timelines. He was sleeping in his hotdog stand. He was joking. He was preparing christmas presents. He was standing under an open sky, letting the rain wash over him as he grinned happily.  He was fighting in a hall made of false sunlight. A fight that went on forever...

He felt himself being torn apart, his mind at the verge of shattering. And then, suddenly, in the haze of images, there was Papyrus. Sturdy and strong. Papyrus was an anchor, his anchor.  All he needed to do was to think of his brother. His brother who was always there for him. Who was kind and generous. Even now he was helping him back from the brink. In his mind’s eye, Papyrus reached out to him, offering his hand. Sans reached up to take it, to get back to his feet.

The flash of a knife. A human in a striped shirt stepped forward. And Papyrus crumbled to dust in front of him. Sans could feel his brother’s dust slip through his fingers. Someone was screaming. It took him a moment to realize that was him.

“Sans! Snap out of it! Your mind cannot survive this!”

Someone was shaking him roughly by the shoulders. Sans blinked. He could still see the dust on his hands, but there was also the shattered lab. Two images overlaying each other, connected by lines of code. He struggled to concentrate on his surroundings.

"Yes, that's it." The voice said. "Tune everything else out."

There was the roar of the Core. The smell of ozone. Yes, he was in the lab, image clear. He was still on the floor next to Cody’s body. But there was someone else now. Gaster. His brother looked the worse for wear. His skull was cracked in several places. And the inky black substance that had come out of the Window was oozing through the cracks. His eyes were clear however. He looked more in control than he had in weeks and his voice was steady. The speech impediment he’d had as long as Sans remembered, was gone.   
Gaster put an arm around Sans and helped him to his feet, much as Sans had done for his brother only minutes earlier. It felt like years ago.

“You were right.” Sans mumbled. And at Gaster’s questioning look, continued. “About the humans. They bring nothing but trouble and death.” He could still see the image of Papyrus crumbling to dust. And that human, the one with the striped shirt and the empty eyes. They were laughing. Laughing as they killed his brother. “I should have listened to you.”

Somewhere in the room an intercom came to life and Alphys' voice squeeked through.

“Doctor Gaster? Sans? Can you hear me? Are you alright? The Core is going critical. I… I am trying to stabilize it from here… but it’s hard. The elevator is out of order. I can’t… kkkkkkggg” Her voice was lost to static.

“Can you stand?” Gaster asked. “You need to get up. You heard her.”

“The elevator is out, bro.” Sans said grimly. “We’re not going anywhere.”

Neither of them gave voice to what they both knew: If the Core went critical, it was going to destroy the lab… But that would likely not be all. Worst case scenario: There might not be an Underground left to escape from.

Gaster closed his eyes, a pained expression on his face. He let go of his brother. “Determination.” He said and he smiled ruefully. “This is what it comes down to. What it has always come down to.” He took a few steps closer to the blazing Core.

Sans lunged after him, ignoring the new waves of pain the sudden movement sent crashing through his skull. He managed to grasp a sleeve of his brother’s coat. “Gas! What are you doing? Are you crazy? You’ll be incinerated!”  
  
Gaster genty took Sans' hand from his sleeve. “The Determination inside of me will allow me to survive long enough to calm the magic of the Core. I can contain the damage to a small area. Not small enough to spare you entirely, but you will survive.”

“I don’t care! What about you?”

“I’ve seen so many outcomes to this. Seen so many different versions of this moment. Trust me. I’ve tried. This is the one thing that can not be avoided. I will always meet the same fate. But I can save the Underground. And save you.”  
A wave of emotion came over his face as he looked at his brother.  
“Listen to me Sans. This might be my last chance to tell you this. You are not the one who was wrong about the humans. I was. They are not good or evil. They are like us. They have tremendous power, but in that power lies the freedom to choose who they become. Sometimes they simply need a nudge. A bit of kindness. You were not wrong. Other times they need someone to confront them with their actions. Someone to judge them when they trespass or abuse their powers. Someone to stop them when they go too far.”

Before Sans could respond, he continued:  
“After today, you will be changed. You might be the only one who can see them for what they are.” A wry smile played around his mouth. “I don’t envy you. But then, I don’t envy myself either.”

A shudder ran through the Core, shaking the entire room. Glass shards that had been lying on the floor began to float in the air. Sans could feel the energy pulling on them, pulling on him.    
“Ah, but I believe I’ve been monologuing again. I do that.” Gaster returned his determination-filled gaze to the Core. “Enough procrastinating. No use in delaying the inevitable…”   

He began to walk towards the blazing orange light. The energy from the Core grew stronger. His coat whipped around him as if he was walking into a storm.

Sans had to cling to an upturned table, in order not to be swept away himself. He chanced letting go of one hand in order to reach out towards his brother. “Don’t go!” He shouted. “I need you! Papyrus needs you!”

Gaster kept going. The sound of his voice carried over the blazing roar in the room. “Papyrus won’t remember anything. But you, brother; you will.” there was a deep sadness in Gaster’s gaze as he locked eyes with his brother one last time. “and for that, I am. truly. sorry.” The storm was tearing into him now. Gaster stopped fighting it. The fragile scientist was swept away, vanishing into the blinding light.

The Core shuddered and exploded with magical energy.

Sans could feel his very soul being ripped apart as the shockwave washed over him. He’d been expecting heat, but it was cold. Ice that burned and tore at his very being. It wasn’t just killing him. It was taking away pieces of him. Evaporating them like raindrops in a forge. His soul was being destroyed. Rendered asunder; bit by agonizing bit.

But it refused.

Suddenly they were dancing around him. Two bright lights; one cyan and one golden yellow. They enveloped him in their glow.  
His left eye showed him a different sight from his right. There were the lights circling around him, but there were also two children. Cody and a girl who looked much like him, orange hair whipping around her face. They embraced, first each other, then him, the three of them holding fast against the storm that raged around the room. Their souls touched, for one moment being one and the same. The pain vanished. In its place was a deep warmth and something else. Forgiveness? A pure and complete feeling of acceptance and love.  
  
The burning energy that was all around them continued to flow for a moment longer and then reversed. It was pulled inwards, back into the Core. The room shook. The children held tightly onto Sans and he hugged them back, clinging to each other as if they were the only three souls in the world.  
  
He must have blacked out for a moment then. When he came to, the Core was back to normal, humming softly, almost peacefully. No more tendrils of magic that pulled at him, no more storm, no more blaze. The orange glow was almost peaceful.  
He sat up, rubbing his head and groaning. He felt as if all his bones had been shattered and roughly been put back together.    
  
“Gaster? Kids?”  
  
There was no one there. The room was a wasteland of broken lab equipment and shattered glass. One wall was miraculously whole. It was the wall that had held the soul containers. Most of them were destroyed, but a few remained standing. No wonder, the king had given specific orders for them to be near indestructible. And two of them were full. One filled with cyan light, the other gold.

Sans stared. Had the children entered them voluntarily? He walked over and touched each of the jars with one of his hands. They were warm to the touch. He could feel the souls resonate with his own. They felt… peaceful.

Alphys voice, distorted by the half-melted intercom took him out of his reverie. She sounded like she was on the verge of a breakdown. “Sans? Sans! Are you there? Are you dead? Say something if you’re dead! I mean, obviously you can’t say something if you’re dead. But say something anyway? … please?”

Sans made his way over to the elevator. He pressed the “talk” button, hoping against hope it’d work.

“Alphys? I’m here.”

There was a relieved gasp on the other side. “Sans? I’m going to get you out of there. It’s just… all the equipment is broken and… the elevator won’t work and...”

Sans closed his eyes and concentrated. He could see so much now. See all the connections. See the code that connected this place to the hallways upstairs. He knew how to get there. He knew how he could summon Gaster Blasters.  Why had that ever seemed difficult? He idly wondered where the limit of his newfound abilities lay. A simple change of a few parameters… He stepped forward. When he opened his eyes, he was standing in the upper hallway of the laboratory. Alphys was facing one of the machines, her back turned to him as she frantically typed away at one of the terminals. “Come on, come on!”

“Hey Alphys.” Sans walked up to her and touched her shoulder. She jumped so high she nearly bumped her head into the ceiling.

“Sans? I thought you were downstairs… and ohmygod… are you alright?”

Sans groaned. Everything hurt, his head was still throbbing and that teleport had taken more out of him than he thought it would. Overconfidence might very well run in the family.  “I don’t feel so good.” He admitted, slumping against the wall.  
Alphys helped him sit down. “What happened?”

“It’s Gaster... “ he said, choking on the name. “He’s gone.”

She gave him a blank stare. “Who?”

 

**Notes for the Chapter:**

> There you have it. This chapter and the one before, certainly were intense to write.  
> Fun fact: When I was done writing this, I realized I pretty much wrote Sans' superhero origin story!  
> A moment of silence for everyone's favorite mad scientist: Doctor Gaster. Here's a song in his honor: https://www.youtube.com/watch?v=hDCv57KYGKA  
> He will be missed (by those that still remember him). But of course he's not entirely gone, or is he?
> 
> More about that in the epilogue! Coming soon to an Archive near you!


	7. Don't forget

_I am everywhere, I am nowhere. I feel the void tearing at my soul. I am in pieces. I can see it all. All the universes, all the possibilities._  
_Brother, father, creator, tormentor. Change a few parameters and everything is different. I am a benevolent guardian. I am the boogeyman monster children fear. My awareness reaches out to so many places, so many realities._  
_I can even hear you. You who are listening to my words right now.  Don't you know who I'm talking about? Come now. I've seen you. Soaking up these tales, these realities, one after another. Always moving on. Never satisfied. You are like me._  
_Don't look so surprised. I told you I am everywhere._

_… I am nowhere._

 

Sans is slumped against a wall. He looks terrible. Ignoring the rules of time and space will do that to you. Alphys is talking, but he’s not listening. His eyes are glazed over with shock.

“Sans, I really don’t know who you are talking about…. There...there hasn’t been a royal scientist for a long time. It’s just you and me here.” she looks worried, and a little scared. “You need a d.. Doctor..”

“Gaster’s a doctor. Doctor Gaster? You know him. My brother!” Sans looks like he’s about to shake her. She backs away in fright at his strange behavior.

“I didn’t know your brother is a doctor.” she says softly.

Sans holds a notebook out to her. It’s filled with lab notes.

“Here! He wrote this!”

Alphys looks close to tears, but she takes the book anyway. She adjusts her glasses and studies a random page, then another. “I can’t read this. It’s some kind of code. It’s all hands and skulls and wait… it feels like I should know this. Something about rain?”

 

 _She does not remember, but she comes close. I am still there, in the far reaches of her mind. Like a forgotten song. Only a few scattered notes remain.  
_ _Interesting._

_“You’re going to have to keep an eye on her”._

Sans spins around at the sound of my voice. “Did you hear that?”

Alphys shakes her head… “Sans, you were at the site of a magical incident. I think you need a checkup. And may… maybe a vacation.”

As soon as Alphys turns her back, Sans flees. He folds space in on itself and appears in Snowdin, in front of his house. He stumbles, his last energy reserves spent. It takes him a moment to steady himself, adopt his usual facade of nonchalance. He knocks. When Papyrus opens the door, Sans presses a piece of paper into his hands. Three skeletons look up from the drawing, waving cheerfully. “Hey Paps, quick question; who did you draw here?”

“ … SANS???” the larger brother stares at the smaller in amazement and with no small amount of happiness.

“HOW DID YOU GET HERE? I THOUGHT YOU WERE STILL WORKING! WASN’T I SUPPOSED TO COME VISIT YOU?”

Sans lets his brother talk for a moment, catching his breath. Then he points back at the drawing.

“What did you draw here?”

A confused look spread across Papyrus’ features. “DID I DRAW THAT?” He frowns in concentration, but eventually admits.  “I DON’T REMEMBER DOING THAT AT ALL.”

“But do you see what it is?”

“OF COURSE! THREE PEOPLE! QUITE A GOOD DRAWING IF I HAVE TO SAY SO MYSELF. THAT ONE LOOKS A BIT LIKE YOU. AND THAT HANDSOME SKELETON IN THE MIDDLE IS CLEARLY ME.” He peers at the image of me. “THAT THIRD ONE IS HARD TO MAKE OUT. WOWIE, LOOKING AT IT KINDA MAKES MY HEAD HURT.”  
  
“You don’t know who it could be?”

“NOPE, SORRY, NO CLUE. LOOK SANS, I’M HAPPY TO SEE YOU, BUT DID YOU COME ALL THIS WAY TO SHOW ME A DRAWING?”

He hands the drawing back to Sans.

“It’s important Paps. You know the person in the drawing… You have to…”

“DRAWING? WHAT DRAWING?”

Sans shakes the drawing in front of his face. “The one you drew! This one!”

“DID I DRAW THAT?” Papyrus frowns in concentration.  “I DON’T REMEMBER DOING THAT AT ALL...”

Sans slumps to the ground. He looks shattered. His brother carries him inside, a worried expression on his face.

 

  
_In the end it always leads to this. The Core is always waiting for me and Sans is left behind, the only one able to see in a world where everyone else is blind._

“Hey Papyrus. I brought you a burger.”

Papyrus looks up from the piece of paper he is scribbling on. It’s full of O’s and X’s.

“THANK YOU. I WAS SO BUSY WITH THIS PUZZLE I FORGOT TO GET FOOD!” Papyrus begins unwrapping the Grillby special.  
  
“I don’t think I’ll go back to working in the lab.” Sans says casually.

His brother frowns. “WEREN’T YOU HAPPY WORKING THERE?”

“I don’t think there’s anything left there for me.”

Papyrus gives him a critical look, putting the burger away, untouched. “YOU KNOW YOU CAN ALWAYS TALK TO ME, RIGHT, SANS? BROTHERS SHOULDN’T HAVE SECRETS.”

There is a lot brothers shouldn’t do. Brothers shouldn’t vanish from existence. “It’s just all this science stuff… It’s rough sometimes, Paps. Sometimes it feels like I know too much. I’m sorry, that probably doesn’t make much sense.”

“NONSENSE. IT MAKES PERFECT SENSE. I USED TO THINK I KNEW VERY MUCH ABOUT SPAGHETTI. MORE THAN ANYONE ELSE IN SNOWDIN. AND BECAUSE I KNEW SO MUCH ABOUT IT, I COULDN’T TALK WITH ANYONE ELSE ABOUT IT. NO ONE SHARED MY PASSION FOR PASTA. AT LEAST UNTIL I MET UNDYNE. CARING VERY MUCH ABOUT SOMETHING NO ONE ELSE KNOWS ANYTHING ABOUT IS LONELY.”

“That’s…” Sans looks surprised “... actually pretty accurate.”

“DON'T BE LONELY, BROTHER.”

Papyrus gives him a tight hug. Sans returns it, gratefully. He winks. “I think you were right. I was working too hard. It’s time to take a break.”

His brother's expression turns stern, though there is still an amused twinkle in his black eyes.  “DON'T TAKE TOO MUCH OF A BREAK THOUGH, I WOULDN’T WANT YOU TO BECOME LAZY. DON’T THINK I HAVEN’T NOTICED YOU SNEAKING AWAY TO TAKE NAPS LATELY.”

“You callin’ me a lazybones? I’m offended, Paps.” Sans smiles.

“I KNOW! YOU CAN HELP ME WITH MY PUZZLES. WE NEED TO BE PREPARED FOR ANY HUMANS THAT TRY TO COME THROUGH.”

“Oh, we’ll be ready.” Papyrus doesn’t notice the flash of cyan and gold that comes and goes in his brother's gaze.

 

 _  
_ _Are you still here because you’re looking for closure? I'm afraid you'll find none here. For my brothers the worst is yet to come._

  
Alphys is sweeping the floor of one of the many rooms. The place is such a mess. Everywhere in the lab are parts of unknown machines and notes in a strange language she does not remember writing. Did Sans write them? No, that does not seem right. She can't help but feel like she's forgetting something. Something important.  
Maybe it's just the silence getting to her. Sans quit a few weeks ago, so she's the only one left working here. She expects king Asgore will promote her to royal scientist any day now. She's not sure she is ready for that kind of responsibility. It's all very intimidating.

Maybe she should make herself a lab assistant. A robot, like she's seen in her anime. But that seems a bit ambitious for now. Better to start with something small... some flowers might go a long way towards cheering this place up. She’s seen some pretty yellow ones thriving in the ruins. Come to think of it, they might have some interesting scientific properties, for them to be able to survive in the underground.  
Her broom sweeps up a piece of paper that was lying crumpled on the floor. She picks it up. The symbols on the page make her head swim.

Yet there's something familiar about the way they’re arranged. She feels that if she looks at them just right, she can make the letters out. Like one of those magic pictures you just need to squint at to see. She shakes her head. It's not working. But she's not deterred. She'll break this code. It's just a matter of time.

 

_  
I told Sans to keep an eye on her, but he never did listen to me. And these days I don't think he can even hear me anymore. Not beyond the few brief messages I manage to slip out between the cracks._

  
Sans sits with his back to the giant door to the ruins. It’s where he goes when he wants to be alone.  
He is holding a drawing. It has three skeletons on it, waving happily. He clutches it tightly, as if it can vanish at any moment.  
He takes a pen and turns the drawing around. On the back he writes:

_“_ Don’t forget.” 

The letters looked small and feeble. It isn’t enough. He traces them, pressing the ink deeper into the paper.

“Don’t forget.” 

And again.

“Don’t forget.” 

Eventually he has added so many layers that they almost push through on the other side. Only then does he stop.

He sighs. He has so many questions and no one to ask them to.

What will he do if he encounters another human? He tried kindness. And not only did he fail to save a single human, it nearly destroyed part of the Underground and got his brother erased from existence. Not really a good track record. And Cody was a good kid. What if someone truly evil comes through? Humans have a tendency to cause chaos. The images that assaulted his mind in the lab have faded, but he still has a faint memory of a child in a striped sweater.  
“Hey Gas…” he says to no one in particular. No one at all. “... do you think everyone can be a good person, if they just try?”

He thinks he hears an answer faintly echoing through the trees. Or perhaps it’s just the wind.

 

  
_I am beyond most emotions now, but I still watch over Sans and Papyrus. My last vestige of sentimentality. There's not much left of my soul. It is fragmented, scattered between worlds. So while I still have at least a semblance of myself, of who I used to be, I've got a question for you._  
_A request, really._  
_If you ever meet my brothers, please treat them kindly._

_..._

_What am I saying? You've already met them, haven't you?_


End file.
